“The present review did not find convincing evidence that these assumptions are valid. There is, however, also no strong evidence that the assumptions are incorrect and/or that there is antagonistic activity between strains in a combination.”

Several factors muddy the waters: total dose variations, too many strains in mixtures and lack of placebo.

Multi-strains may have higher total CFUs. For example, in a 2011 review addressing comparative usefulness, researchers found that based on a limited number of studies, multi-strain probiotics worked better than single strains but it was unclear whether it was a result of synergy or merely a higher dose.

One year later the same researchers, who hailed from the University of Reading in the United Kingdom, wondered if probiotics in mixtures would inhibit each other thereby making them less effective.

They first tested 14 single-species probiotics in that regard. Then 15 single-species and 5 mixtures were assessed against three pathogens. As expected, different species performed diversely. But interestingly, in spite of inhibition when served up together, the fight against pathogens in half the cases was better than if each had gone solo.

“…in many cases a probiotic mixture was more effective at inhibiting pathogens than its component species when tested at approximately equal concentrations of biomass.”

Results? Probiotic mixtures prevented the growth of urinary tract pathogens but were not significantly more inhibitory than single strains.

What to make of the contradictory outcomes, albeit in limited and dissimilar conditions?

More structured research is necessary to tease out the facts, write Ouwehand and colleagues in their paper.

“For human, animal, and in vitro studies, it is recommended to work with the simple setup of 2 single strains, a 2-strain combination, and placebo. It is also important in such research to take into consideration the doses, as a combination product will have a higher total dose.”

Only then, if the research bears it out, can it be decided that more is actually better.